[PATCH V4 19/27] selftests: ublk: fix user_data truncation for tgt_data >= 256

Ming Lei
2025-11-21T02:00:27Z

The build_user_data() function packs multiple fields into a __u64
value using bit shifts. Without explicit __u64 casts before shifting,
the shift operations are performed on 32-bit unsigned integers before
being promoted to 64-bit, causing data loss.

Specifically, when tgt_data >= 256, the expression (tgt_data << 24)
shifts on a 32-bit value, truncating the upper 8 bits before promotion
to __u64. Since tgt_data can be up to 16 bits (assertion allows up to
65535), values >= 256 would have their high byte lost.

Add explicit __u64 casts to both op and tgt_data before shifting to
ensure the shift operations happen in 64-bit space, preserving all
bits of the input values.

user_data_to_tgt_data() is only used by stripe.c, in which the max
supported member disks are 4, so won't trigger this issue.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
tools/testing/selftests/ublk/kublk.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/ublk/kublk.h b/tools/testing/selftests/ublk/kublk.h
index fe42705c6d42..38d80e60e211 100644
--- a/tools/testing/selftests/ublk/kublk.h
+++ b/tools/testing/selftests/ublk/kublk.h
@@ -220,7 +220,7 @@ static inline __u64 build_user_data(unsigned tag, unsigned op,
_Static_assert(UBLK_MAX_QUEUES_SHIFT <= 7);
assert(!(tag >> 16) && !(op >> 8) && !(tgt_data >> 16) && !(q_id >> 7));

- return tag | (op << 16) | (tgt_data << 24) |
+ return tag | ((__u64)op << 16) | ((__u64)tgt_data << 24) |
(__u64)q_id << 56 | (__u64)is_target_io << 63;
}

--
2.47.0